[dice]

Lettuce is not normally used, because farmers are not
generally growing a winter cover crop to harvest it. They
are growing it to replenish organic matter in the soil,
add nitrogen, bring up minerals from down deep in the soil,
provide mulch, provide habitat for beneficial insects, etc.

You could use lettuce, but it roots kind of shallowly and
does not leave much behind when you harvest it. In a
container mix, peas or beans would probably be better
for a cover crop that you can eat (more roots, that happen
to fix nitrogen on them, and a tendency to exude enzymes
that free up phosphorus from insoluble phosphorus compounds).
You want "inoculated" seeds for this (inoculated with the
kind of microbes that grow on the roots of that specific kind
of legume and fix nitrogen in nodules on the roots).

Mustard is a good one for providing lots of nutritious
organic matter in the top growth to mix back into the
soil (container mix) or into compost.

In your climate, you can probably grow almost any cool
season vegetable as a winter crop. Beans and peas just
do more for the soil than most and some happen to be
edible and tasty.